Electrical heating systems are for example used in home technology as heating plates in ovens, in hair dryers and fan heaters, in motor vehicle technology for heating the supercharged air or the catalysts for gasoline or diesel fuel, and in chemical technology at reaction columns. Normally the temperature is measured and the measured value is used, via a closed loop, to control heating elements in order to achieve the desired target temperature.
The heating conductors in use are normally heating conductors based on ferritic FeCr (AL)—or austenitic NiCr (Fe)—alloys. However, these materials are rather difficult to use for a more finely tuned temperature because of the low temperature dependence of their specific resistance.
Co alloys with additions of Fe, Ni and similar metals have proven their worth for spark plugs in combustion engines, but they are also quite expensive because of their high Co content. In addition, the specific resistance value is very low in such materials, making it necessary to add other resistors for a better adaptation to the internal resistances of existing power sources.
DE 3825012 A1 describes a control element for a spark plug consisting of a cobalt-iron alloy with about 20–35% iron. This alloy morphs into a cubic area-centered crystalline structure beyond about 1,000° Celsius which, because of the frequent temperature changes and the associated phase transformations, leads to the material's thermal fatigue.
According to EP 0523062, cobalt-iron alloys with an iron content of 6–18% that avoid the effect of thermal fatigue are also used. These alloys have the advantage of a high temperature factor, but their disadvantage is the high price.
The use of pure Fe leads to a pronounced temperature-dependence of the resistance, but there are problems with phase transformations in the interesting temperature range above 900° Celsius. This leads to a hysteresis behavior resulting in an ambiguous temperature resistance curve and in thermal fatigue that can lead to rupture.
In addition, at low temperatures and/or low heat volumes the temperature-dependence of a single resistor is currently also used, which in turn itself serves as heat resistance.